1. Field of the Invention
The present invention pertains, in general, to preparation methods of exfoliated nitropolymer/silicate nanocomposites and, more specifically, to a method of preparing a nitropolymer/silicate nanocomposite having an exfoliated structure using a pure silicate which is not organically modified.
2. Description of the Prior Art
Compared to conventional microcomposites, polymer/silicate nanocomposites exhibit excellent mechanical properties and dimensional stability as well as decreased permeability to various gases even though silicate is added in a small amount. In addition, polymer/silicate nanocomposites have superior thermal stability and self-extinguishing property. The reason for these properties is that individual layers of silicate dispersed uniformly in polymer matrixes exist in plate-like shapes, and thus have high aspect ratios and large surface areas capable of interacting with the polymer.
Due to such superior properties as described above, the polymer/silicate nanocomposites have a variety of industrial applications. Practically, commercialization of the nanocomposites is progressing in USA, Europe and Japan. For instance, a nylon-layered silicate nanocomposite is applied as a timing-belt cover of automobiles by Toyota Group, Japan.
Polymer/silicate nanocomposites are classified into intercalated forms and exfoliated forms. Intercalated nanocomposites have structures wherein spaces between layers of silicate are widen due to the polymer intercalated between galleries of the silicates, while regularly layered structures of the silicates are maintained as they are. Meanwhile, exfoliated nanocomposites have structures wherein each layer of silicate is dispersed in a polymer matrix to the extent of completely disrupting the inherent regularity of silicate layers. The exfoliated nanocomposites have been known to be superior to the intercalated nanocomposites in physical properties. It has been surmised that this is due to the increased probability in the exfoliated form that polymer chains may contact silicate, provided that equal amount of silicate is used, which, in turn, enhances physical properties of the composite, such as modulus, self-extinguishing property, and dimensional stability.
However, the polymer/silicate nanocomposites are difficult to prepare. The silicates, which have hydrophilic surface and high attraction between layers, are hardly mixed with a hydrophobic polymer, and thus intercalation of the polymer cannot be expected. In conventional techniques, alkylammoniums are used to modify the hydrophilic surface of silicates to make them hydrophobic, and hydrophobic polymers are intercalated between the layers of modified silicates, to provide a nanocomposite.
There has been suggested various methods for intercalating polymers between layers of organo-modified silicates. For example, polymerization may be performed immediately after intercalating molten monomers or dissolved monomers into organo-modified silicates, to obtain nanocomposites. Alternatively, in a solution-intercalation method, a polymer may be dissolved in a solvent and then intercalated between layers of layered silicate. Finally, silicate may be added to a molten solution of crystalline polymer to prepare a nanocomposite.
All the methods mentioned above share a common feature of using organo-modified silicates. However, alkylammoniums, which are used for modification of surfaces of silicates, detrimentally affect thermal and mechanical properties of final nanocomposites. Hence, improvement of physical properties of the composite is made within the limit which a small quantity of organo-modified silicate permits. Further, excessive alkylammonium may migrate to the surface of the nanocomposite, rendering the nanocomposite harmful to the user. Moreover, in manufacturing installations, additional cost is required for making organo-modified silicates or for purchasing commercially available organo-modified silicates. Therefore, production cost of a final nanocomposite is increased, thus weakening the market competitiveness of the nanocomposite.